Anthony Fauci, director of the National Institute of Allergy and Infections Diseases in the U.S., said the procedure was too costly and too dangerous to employ as a firstline cure. But he said it could inspire researchers to pursue gene therapy as a means to block or suppress HIV.

"It helps prove the concept that if somehow you can block the expression of CCR5, maybe by gene therapy, you might be able to inhibit the ability of the virus to replicate," Fauci said.

David Roth, a professor of epidemiology and international public health at the London School of Hygiene and Tropical Medicine, said gene therapy as cheap and effective as current drug treatments is in very early stages of development.

"That's a long way down the line because there may be other negative things that go with that mutation that we don't know about."

Even for the patient in Berlin, the lack of a clear understanding of exactly why his AIDS has disappeared means his future is far from certain.

"The virus is wily," Huetter said. "There could always be a resurgence."

(This version CORRECTS spelling of doctor's name to Huetter throughout.)